1. Field of the Invention
The present invention relates to a method and apparatus for converting motion image data and also to a method and apparatus for reproducing motion image data. More particularly, the present invention relates to a method and apparatus for converting motion image data such that image data output from a single-plate solid-state color image sensor is compressed depending on the amount of motion measured for each block of the image data into a form that allows it to reproduce high-quality image data having no significant degradation in image quality, and also relates to a method and apparatus for reproducing high-quality motion image data.
2. Description of the Related Art
A single-plate solid-state color image sensor has a color filter disposed on the surface of each pixel of the image sensor such that only a particular wavelength component of light can pass through the filter and reach the image sensor. The color of each point of an image is represented by a combination of color components sensed by a plurality of pixels. The color filters disposed on the surfaces of the respective pixels have different colors assigned thereto depending on the positions of the pixels. FIG. 1 shows an example of a color pattern of the color filter array. In this example shown in FIG. 1, each color of red (R), green (G), and blue (B) is represented by a set of four pixels. The color pattern shown in FIG. 1 is called a Bayer array. In the single-plate solid-state color image sensor, as described above, each pixel provides information of one color selected from R, G, and B. The other missing color components at each pixel are acquired by interpolation from color information provided from neighboring pixels. This process of reproducing necessary color components to acquire a complete set of color components for each pixel is called demosaicing.
FIG. 2 shows an example of an image pickup apparatus including a single-plate solid-state color image sensor. Of various color components of light passing through an optical lens 1, only a particular color component passing through a color filter 2 reaches the single-plate solid-state color image sensor. The solid-state image sensor 3 converts an optical image into an electrical signal and outputs the resultant image signal in the form the electrical signal. The output image signal is converted into a digital signal by a digital-to-analog converter (not shown), and supplied to a camera signal processor 4. In the camera signal processor 4, the image signal is subjected to processing such as clipping, a gamma correction, a white balance correction, and demosacing. The resultant image signal is supplied to a motion image compressor 5. The motion image compressor 5 compresses the data size of the motion image signal and outputs the compressed motion image signal in a predetermined motion image format. A recording unit 6 records the motion image data in the compressed form on a recording medium. The compression of motion image data is not necessarily needed. However, in recent years, a great increase in the number of pixels of the image sensor has been achieved, which results in an increase in the data size of image data. To deal with motion image data with such a large data size, it is common to compress the motion image data. The data compression is also useful to achieve a reduction in the apparatus size.
For example, when motion image data is stored on a storage medium such as a flash memory, a hard disk, or DVD, or when motion image data is transmitted via a network, the motion image data is compressed to reduce the data size thereof. An increase in quality of motion image data has been achieved in recent years, for example, by a HD (High Definition) technique. However, the result of the improvement in data quality is an increase in the data size. In view of the above, intensive research and development activities are being carried out to improve the compression ratio in the compression of motion image data and minimize the degradation in image quality that occurs when image data is reproduced by decompressing the compressed image data.
A known method of compressing motion image data is to decimate pixels included in each frame of motion image data. This method is referred to as spatial decimation. Another method is to decimate frames (the frame rate is reduced as a result of frame decimation). This method is referred to as temporal decimation.
By performing such data conversion, the data size can be reduced and thus it becomes possible to efficiently store data on storage medium or transmit data via a network. However, compression of data causes degradation in image quality. That is, data reproduced from compressed image data is not as good as the original data. The problem with degradation in image quality is serious, in particular, when original data is of a high-resolution image.
A wide variety of techniques have been proposed to reduce such degradation in image quality. For example, Japanese Unexamined Patent Application Publication No. 2003-169284 discloses an image compression technique in which parameters are set based on information indicating the brightness of an image, and a compression mode is switched depending on the brightness of the image. Japanese Unexamined Patent Application Publication No. 2002-27466 discloses an image compression technique in which a screen is divided into a plurality of regions, and an optimum compression mode is determined for each region.